Seems like a good idea to me.

jms452 - I am going through the same thought processes as you! We are in a detached dormer bungalow in which I have installed a lot of insulation, both in the attic and IWI upstairs. Although there are 4 radiators in the rooms upstairs they are permanently off as the heat rising from downstairs keeps the upstairs at around 20C. As we spend most of our time either in the kitchen/diner or the lounge I am thinking of an air-to-air ASHP just to supply the lounge and kitchen. A split unit would be good, maybe one blower unit in the lounge and one in the hallway.

A friend of ours has had a Worcester Bosch Greensource air-to-air unit for many years. He is in the same situation as you i.e. has gas C/H and uses the Greensource when there is no requirement to have the entire C/H system working. It is situated in his hallway and by leaving the kitchen and lounge doors open, heats those rooms too. It works well and I think would definitely be the sort of thing you are looking for. I think his is 2kW but I can check if you are interested.

We have a wood pellet boiler which does C/H and DHW (the latter only for about 7 months a year as we have solar thermal). I have been disappointed with this (for many reasons!) and if I had my time over I would not have opted for biomass. RHI payments will stop next October, hence now looking at ASHP's. There is a huge choice of air-to-air systems from what I can see already but my main dilemma is to find a long established local installer who would come and take a look at our place and offer independent advice! One question I am wrestling with is do I go for a multiple split air-to-air system or do I bite the bullet and install a full blown air-to-water ASHP system.

Go for it; I've had my Toshiba R32 inverter ducted system for over a year now, and I would not go back to any form of wet CH.
Heating the air direct instead of the intermediate stage of first heating water seems the way to go for me. Also, to select an A2W heat pump just to get the perceived added benefit of some DHW spin off seems equally pointless. You can directly heat the water more efficiently with electricity, especially if you have PV to supplement your grid consumption.

If you're considering going down the heat pump route anyway it may be worth looking at a slightly larger model with split capacity and putting in two or more indoor units. The extra hardware costs may not be too great.

As for which manufacturer to go for; there are loads. My advice would be to stick with the established makes, but your choice of F gas fitter may have some bearing, as they all have their particular likes often merely based on the amount of units they fit and, I guess, the trading discounts they receive rather than any pluses and minuses of the units themselves. The aesthetics of the indoor units is also very important, as is their noise output. They vary too with the sophistication of their air distribution. I don't know how the indoor unit/s will impinge on your MVHR but the room siting may need careful consideration.
If you want to have some DIY input and you're reasonably handy find an independent fitter who would be prepared to let you do some of the legwork,- it's what I did. This could include preparing an outdoor hard standing, basic cable runs etc. and in my case trunking layout and condensate drains. That just left the fitter to put in the copper refrigerant pipework, with my help, and testing and commissioning. Your fitter may even let you buy all the stuff yourself, mine did.

https://www.worcester-bosch.co.uk/img/rac_assets/erp/single_units/Single_Split_2.6kW_Product_Data_Sheet.pdf

COP at -7C/20C = 3.0

Now you've got me thinking Jeff. I have no experience of Economy 7 or what the rules are for that tariff but I will look into it.
At the moment I'm using the A2A as principle heating for the rear of my home, with the occasional log fire, ( simply because I like it and the wood is free).
This leaves my biomass logwood boiler to provide heat for the hallway, bathroom and any other room on an, "as and when required" basis, plus DHW via a plate heat exchanger. I can alter the latter again as and when required, onto pure Solar with a simple change lever. The Solar tank also acts as a pre-heat on the PHX supply side.
If I decide to push ahead in future with another A2A it will most likely be one or two wall hung units in the hall and bedroom end of the house.

Hi Dave, Thanks for the explanation and it may be something to look for use with resistive underfloor heating in for instance bathrooms and possibly a storage heater in the hallway, both of which I could easily wire onto a single timed circuit as well as the A2A of course which is already on its own breaker.

We don't have any separate circuits, I just use timers on things wired into the normal circuits. I think there is the possibility of using a separate switched circuit from the meter but I don't remember the details I'm afraid.

djh - yes, I can concur that although the night time tariff is about one third the daytime cost I have yet to ascertain what the daytime cost will be. As I am thinking of changing electricity supplier, this will be one of the things I will be looking out for. I wouldn't want to be paying OTT for daytime usage whilst only making a small saving on Economy 7!

Posted By: owlmanHi Dave, Thanks for the explanation and it may be something to look for use with resistive underfloor heating in for instance bathrooms and possibly a storage heater in the hallway, both of which I could easily wire onto a single timed circuit as well as the A2A of course which is already on its own breaker.

I'm not so sure that electric UFH in the bathroom is something that you would want to put on a might time only supply, you would get a nice warm bathroom about 3 am....not really when you want it. An electric storage heater, well that's a whole different matter.

Posted By: owlmanHi Peter, Does Economy 7 tariff change at 3 am or, are all providers different?

No, PiH was just using 3am as a time in 'the middle of the night', I think. E7 periods are almost all between midnight and 8am, but exactly when they start and stop varies a bit, and of course they're only seven hours, not eight.

There's also Economy 10, which has a few cheap hours during the daytime hours and some more overnight. Don't know if you can still get E7 or E10 meters installed, but you can get a free smart meter which will work with an E7/E10 tariff.
https://selectra.co.uk/energy/guides/tariffs/economy10#what-is-an-economy-10-meter

Other emerging Smart Meter tariffs range from one with 21 hours of slightly-cheaper electricity throughout the daytime and nighttime with 3 expensive hours in the evening peak. At the other end there's offers with 5 very-cheap hours in the wee small hours. Some give you more cheap hours at the weekend.

The UK smart meters do not physically switch anything on themselves (due to data privacy etc), instead you set your own timeswitches or smart plugs or hive-type thermostats to come on the cheapest time. Or just turn stuff on yourself when you want it, and be billed at the prevailing rate for the hours you have it on.

My expectation would be that E7/E10 will disappear in the next few years to be replaced by variable tariffs which reflect demand and renewable generation so it's probably worth at least thinking about options for control more flexible than simple time switches spread round the house plus means to store heat in various forms (water tanks, slabs).

Posted By: Ed DaviesMy expectation would be that E7/E10 will disappear in the next few years

I think it may take longer than that just because of inertia. Who's going to force me to get my tariff and meter changed?

options for control more flexible than simple time switches spread round the house plus means to store heat in various forms

The Timeguard switches have four switchable periods a day and can switch weekdays and weekends separately, so they're fairly flexible. But I've replaced the timer I used with my MVHR post heater with a remote-controlled plug that I control with cron jobs run on my emonpi system. So it's not too difficult. As you say, the ability to store the electricity as heat in a water tank or in the the structure of the building (or a storage heater!) is key to making it work/pay. Battery prices aren't yet at a level where they're a good alternative IMHO.

Posted By: djh Who's going to force me to get my tariff and meter changed?

Posted By: djh elsewhere re: electricity price increases night rate +18% !!!

It's perhaps worth changing tariff yourself before anyone forces you to - if you stay on a legacy tariff for too long then the price will only go in one direction....! Inertia can be expensive, but for the same reason, I agree that the energy co will want to keep people on the legacy tariffs for longer than might reasonably be expected.

With the increase in car charging, electricity prices in the evening peak are likely to increase, to ration usage. So Time of Use tariffs should become more attractive, possibly replacing E7/E10.

How about https://www.orionairsales.co.uk/ducted-air-conditioning-28-c.asp ? Presumably they don't heat the air as hot as a direct electric heater does (to keep the CoP up) so you need to move more air?

Posted By: djhBut I've replaced the timer I used with my MVHR post heater with a remote-controlled plug that I control with cron jobs run on my emonpi system.

Yep, something like that that looks at the time-of-use tariffs for the next 24 hours and decides when to heat is what I had in mind. I.e., don't build in the time clocks too much; you want to future-proof by arranging for all the high energy use things to at least have some way to be modified to be controlled remotely without too much cost or disruption.